Fossil fuels may be carbon-spewing energy sources but they do excel at storing energy. While wind and solar power are much cleaner, neither wind nor sunlight can be bottled up and stored. The challenge is finding ways to store the electricity they generate, efficiently and on a large scale.

Compressed air stored underwater in tethered balloons. Cameron Lewis of the Toronto-based company Hydrostor and University of Windsor engineering professor Rupp Carriveau explain.

More efficient batteries based on lithium sulphur and lithium air chemistry, being developed by Linda Nazar, a chemistry professor at the University of Waterloo.

Lithium-ion batteries for electric cars being repackaged by the Toronto-based company Electrovaya to power whole communities. Raj Dasgupta, the company's director of research, thinks this could be a big part of his company's future.

A liquid metal battery system based on electrolytic aluminum smelting big enough to power a city, something MIT materials chemistry researcher Donald Sadoway calls a "disruptive technology," far different from small battery technology.